a certain enzyme and a component from within the cell are released upon cutting, and this produces a volatile compound which turns into sulfuric acid upon in contact with the moisture in your eyes. So, the more you tear, the more you get the hit. Using sharp knife to cut onion minimizes the rupturing of the cells.. wetting the knife allows the water in your knife to take the hit, and cutting onion cold slows down the enzymatic reactions. If you are going to cook the onion anyway, another trick is to microwave it until it starts to steam, in deactiving/ destroying/ denaturing, the enzyme.